Low noise operation is an important goal for many circuits. In computed tomography (CT) medical imaging, for example, the noise performance of circuitry used to detect X-rays passing through a patient impacts the precision with which the X-ray dose to the patient can be kept within safe limits. In a typical CT detector circuit, an integrator is used to detect charge generated by a photodiode in response to the X-rays passing through the patient. As the integrator occupies a relatively early position in a circuit chain, its noise performance can dictate, or at least greatly influence, the overall noise performance of the CT detection circuitry.
Several drawbacks exist, however, with using previous integration circuits in noise-sensitive applications such as CT imaging, which also involve the detection and measurement of signals over a potentially large dynamic range. Noise performance is a greater concern at smaller signal magnitudes than at larger signal magnitudes in a given dynamic range. As a result, an integrator designed to provide sufficient full scale performance for larger signals may not provide sufficient performance for smaller signals. Similarly, an integrator designed to provide sufficient noise performance for smaller signals may impose undesirable disadvantages for larger signals.
Thus, a need exists for circuits, including integrators for use in CT imaging, that have improved noise performance over a high dynamic range without unacceptable accompanying drawbacks.